The present invention relates to the production of wax molds for use in casting. In particular, the invention is related to an apparatus and method for measuring the service life of a metal die, used to produce wax molds, as a function of the number of wax injections to which the die is subjected.
Investment casting, specifically, xe2x80x9clost waxxe2x80x9d casting, is an industrial process used in the production of cast parts which involves the steps of: (1) injecting wax or other suitable polymer into a metal die to produce a pattern or mold, (2) removing the mold from the die, (3) coating the mold with a ceramic shell, (4) heating the shell to melt and remove the wax or polymer, and (5) subsequently filling the ceramic shell with molten metal.
The lost wax process begins with the production of a heat-disposable mold. The molds are usually made by injecting wax or other polymer into a metal die. Upon cooling, the metal die is opened and the mold is removed. This process is repeated until the desired number of molds are produced.
Typically, each mold will include one or more gates, which are generally flat or rod-like wax or polymer projections that attach the mold to a sprue. A sprue is a wax or polymer connector used to fasten the molds together to form a cluster.
Next, the cluster is dipped into an investment liquid, typically in the form of a ceramic slurry. The excess slurry is drained off and the cluster of molds is coated with a fine ceramic sand and dried. On successive dips, progressively coarser grades of ceramic material are applied, until a self-supporting shell is formed encapsulating the cluster.
The coated cluster is then placed in a furnace or steam autoclave where the wax or polymer, including the molds, gates, and sprues, melt and flow out of the mold. Since wax is combustible, the wax instead may be heated and burned out, and in such case vents are provided to permit gases to escape. The resulting ceramic mold corresponds to the shape of the cluster, with each mold in the cluster having precisely shaped hollow regions or cavities corresponding to the desired configuration of a finished part. The ceramic mold is then fired to burn out any remaining wax or polymer and to preheat the mold in preparation for the casting operation. The hot mold is taken from the furnace and molten metal is immediately poured into it.
After the metal-filled mold has cooled, the ceramic mold material is removed from the casting cluster by any suitable method; for example, sandblasting, mechanical vibration, or chemical cleaning may be employed. Individual castings are then severed from the cluster and any remaining protrusions left by gates or sprues are removed by chasing or other suitable finishing methods. The casting is then ready for any secondary operations, including heat treating, machining, and straightening.
Investment casting permits a high volume of production from a single metal die through the use of wax or polymer molds. For very large parts, a cluster may carry a single pattern and for smaller parts, a cluster may consist of many individual molds. Investment casting also permits the consistent reproduction of features having a high degree of complexity, and thus the finished parts may be given highly detailed and geometrically complex surfaces. The highly accurate reproduction process can obviate the need for the substantial machining operations that would otherwise be required to impart such detail.
Metal dies, however, have a life expectancy, as do all other casting parts, whereupon continued use of the metal die after a certain number of uses or cycles results in nonconforming castings. Because the metal dies are thermally cycled, fatigue-related failures may occur. In addition, the repeated use and stressing of the dies may result in slight or substantial changes in die dimensions that are outside permissible tolerances for production. In particular, in the case of the lost wax process, changes to the die may result in the creation of wax or polymer molds that do not meet the specifications and tolerances required for the final product.
Various controls have been developed for molding machinery. For example, U.S. Pat. No. 4,208,176 to Salerno discloses a time independent cycle control for plastic injection molding machines. Transducers are employed to monitor pressure and/or temperature within the mold cavity and when these parameters reach predetermined magnitudes, switching from stage to stage of the molding machine cycle ensues.
U.S. Pat. No. 5,571,539 to Starkey discloses a mold with an on-board counter or monitor. The counter is actuated with each opening and closing cycle of the mold to maintain a count of the operating cycles performed. The counter or stroke monitor may be either a mechanical counter or an electrical counter which is incorporated into a mold half to remain with the mold when it is stored away from a molding machine, when operated during an initial set-up operation, and when operated during a production run, and is hermetically sealed. An actuating mechanism is associated with the counter and secured to one of the mold halves so that the actuating mechanism causes the counter to advance and register a count with each molding cycle.
In addition, U.S. Pat. No. 5,361,826 to Yamauchi et al. discloses a laminar flow injection molding apparatus and method capable of directly judging flow mode of molten metal flowing into a metal mold as being either a laminar flow mode or a turbulent flow mode. If the molten metal has laminar flow, the casting operation is continued, and if the molten metal has turbulent flow, valve opening degree of a hydraulic circuit is re-adjusted. Molten metal detection means is disposed at least at one of the runner portion, the cavity and the gas vent passage for generating a molten metal detection signal each time the molten metal contacts the detection means in single injection.
Furthermore, U.S. Pat. No. 5,246,643 discloses a discrimination method for maintenance timing for injection molding machines. During the execution of one molding cycle by means of each injection molding machine, the respective values of various process parameters, such as molding cycle time, are cyclically detected, and the detected parameter values are loaded as candidate monitor data into a memory of a numerical control device. A molding monitor unit intermittently receives the various process parameter values associated with each injection molding machine as monitor data values, and discriminates the occurrence of deterioration of the various expendable parts of each injection molding machine by the input monitor data values.
Despite these developments, there exists a need for an apparatus and method for monitoring the service life of metal dies, and for indicating when a die has reached its life expectancy. There further exists a need for an apparatus and method that permit the monitoring of a bulk production operation to automatically monitor the number of injection cycles that a die has undergone, and to automatically terminate the injecting process when a preset service life has been reached. In particular, there exists a need for an apparatus and method for monitoring the usage of a metal die without manual inspection.
The present invention is related to a die assembly for forming wax patterns. The die assembly includes at least two cooperating mold portions together defining a main cavity, a runner in communication with the main cavity, a temperature sensor disposed proximate the runner, and a counter coupled to a mold portion and connected to the temperature sensor. When wax is injected through the runner into the main cavity, the temperature sensor measures an increase in temperature and the counter registers an injection cycle. In some embodiments, the mold portions are formed of aluminum, the main cavity is at least partially formed of aluminum epoxy and resin, and the main cavity is formed in substantially the shape of a golf club head. The temperature sensor may be a thermocouple, and the counter may be a digital counter. The wax may be injected between about 120xc2x0 F. and about 200xc2x0 F.
The counter may be a totalizer-type counter. An alarm connected to the counter may be triggered when a preset number of injection cycles are registered by the counter. The alarm may be at least one of a visual alarm and audio alarm. The counter may include a display for showing the number of cycles registered by the counter, and also may include a reset function for resetting the number of cycles to zero, a lockout function for preventing unauthorized resetting of the counter, and a memory function for storing the number of cycles registered. The counter may be demountably attached to the mold portion.
A valve may be provided in communication with the runner, the valve having open and closed positions, and when a preset number of cycles are registered, the valve may be closed to prevent further injections of wax into the die assembly.
The present invention also is related to an injection molding method including: injecting a flowable polymer into a mold cavity; sensing mold temperature proximate the mold cavity; comparing the mold temperature to a first preselected temperature and a second preselected temperature, the first preselected temperature being different from the second preselected temperature; and registering a count when the mold temperature reaches the first preselected temperature and subsequently reaches the second preselected temperature.
In one embodiment, the method further includes comparing the number of registered counts to a preselected count limit, and preventing flow of polymer to the mold cavity when the number of registered counts exceeds the preselected count limit. In another embodiment, the method further includes comparing the number of registered counts to a preselected count limit, and activating an alarm when the number of registered counts exceeds the preselected count limit. The alarm may be at least one of an audio alarm and a visual alarm.